Study on CD variation in the vicinity of exposure field edge in EUV lithography

Intra-field CD uniformity control is one of hurdles in EUV lithography. Reflection imaging system intrinsic to EUV causes CD non-uniformity especially in exposure field edge. To analyze dominant contributors to make this intra-field CD non-uniformity in EUV lithography, influence of flare from adjacent fields and in-band and out of band refection from reticle masking blind(REMA) and mask black border were investigated through intensive sampling of CD measurement. Also mask border condition and REMA open settings are split into various settings to find out the impacts from each contributor. Two ASML EUV scanners, alpha demo tool(ADT) and pre-production tool(PPT) are used for the experiment. Fortunately, DUV out of band(OoB), reflection of REMA and the flare from adjacent fields are found to be not significant in NXE3100. The results presented here lead us to the conclusion that the EUV refection from mask black border is the main contributor and CD non-uniformity of the field edge can be overcome through optimized REMA setting.